For AstraZeneca Plc, when it comes to beating cancer there’s strength in numbers.

The drugmaker is combining two medicines to overcome lung cancer’s defenses against an immune system attack, an approach that became more attractive after rivals’ studies showed that single-drug therapy might only work in certain patients. Now AstraZeneca is racing Roche Holding AG and Bristol-Myers Squibb Co. to be first to the market with a two-drug treatment that helps more newly-diagnosed people.

“We made a deliberate choice to go right away to the combination strategy,” Mondher Mahjoubi, general manager of oncology at the AstraZeneca, said in an interview.

Once seen as a laggard in immune-oncology, AstraZeneca’s fortunes got a boost in August when Bristol-Myers’s Opdivo failed an ambitious test that tried to show its effectiveness as a first-line treatment for a broad swath of patients with lung cancer. Results of single drug trials such as this one have shown that while a single treatment is effective on tumors with high levels of a biological marker called PD-L1., when levels of the biomarker are lower, a combination approach is necessary, Mahjoubi said.

Backbone of Therapy

The backbone of AstraZeneca’s approach is durvalumab, which blocks PD-L1, a protein “checkpoint” on some cancer cells that helps them hide from the the body’s immune system. The drugmaker is studying durvalumab in combination with another of its immune oncology drugs, tremelimumab, as a first-line treatment for patients with non-small cell lung cancer, the most common form of the disease. The company aims to file for approval for the combination in the U.S., Europe and Japan next year.

“It is possible, provided that the data supports filing, that we would be first” with an approved first-line combination treatment for non-small cell lung cancer, said Sean Bohen, chief medical officer at AstraZeneca. “There is a huge unmet need, and as a result, a good market opportunity.”

Lung cancer killed about 1.59 million people around the world in 2012, according to the World Health Organization, making it the leading cause of cancer deaths. It’s the biggest killer in the U.S., and there will be an estimated 224,000 cases and 158,000 deaths this year, according to the American Cancer Society.

Competitors

Other checkpoint inhibitors like durvalumab include Bristol-Myers’s Opdivo and Merck & Co.’s Keytruda, and have showed early promise in lung cancer. Cambridge, England-based AstraZeneca has the broadest portfolio of combinations, with 19 combinations of immune oncology medicines in its experimental pipeline, according to Bloomberg Intelligence, though all of the drugmakers are exploring combination treatments in some form or another.

Bristol’s approach is similar to AstraZeneca’s: mixing two immune-oncology drugs. A combination of its Opdivo and Yervoy is in late stage trials slated to complete in early 2018.

The New York-based company’s emphasis on combinations comes after the drugmaker in August failed to show Opdivo was significantly better than chemotherapy in first-line therapy. At the European Society for Medical Oncology meeting in Copenhagen on Sunday, Bristol-Myers presented more data from the trial that showed little difference in how first-line lung cancer patients did on the drug, even in those with high PD-L1 levels.

That failure could disappoint doctors and investors. The results were “below expectations,” wrote John Scotti, an analyst at Evercore ISI in New York.

Roche too is in late-stage trials of its immune oncology drug Tecentriq in combination with chemotherapy, and with chemotherapy and the already approved targeted therapy Avastin, as a first-line treatment in patients with lung cancer. It expects to file for regulatory approvals for the combinations in 2018.

Merck presented data at the conference on Sunday from an early-stage trial of a combination of its checkpoint inhibitor Keytruda and conventional chemotherapy tested on lung cancer patients. The results showed the combination was more effective than chemotherapy as a first-line treatment, regardless of their levels of the PD-L1 biomarker.